Low Density Parity Check (LDPC) codes have recently attracted considerable attention owing to their capacity-approaching performance and low complexity iterative decoding. LDPC codes can be applied to a wide range of applications such as wireless communications, satellite communications and storage.
Currently, LDPC code is considered for in the high throughput wireless local area networks (WLAN), such as IEEE 802.11n, as an optional advanced code to improve system throughput. However, several issues need to be solved to match the LDPC code with unique system characteristics of different WLAN systems. First is the code size. Since LDPC code works better with longer code size, the code size should be selected as large as possible to ensure performance. However, since a WLAN system is a random access based system, the code size is limited by the SIFS (Short Inter-Frame Space) decoding budget. Therefore, the largest code size is limited to around 2K bits. Second, in high-throughput WLAN systems, the transmitted PPDU (Physical Protocol Data Unit) is large, which requires using several LDPC codewords. A method for concatenating the LDPC codeword within a PPDU is an important design issue. Since transported data packets can be any size from typically about 40 bytes up to 12000 bytes and larger, the WLAN system must be able to encode packets with variable lengths in a consistent manner. This consistency is required to ensure that the receiver always knows how to reconstruct the information data from the encoded transmitted data.
In a typical LDPC coded 802.11n WLAN system, the scrambled information bits are first zero padded to integer number of LDPC codeword, then coded with a systematic LDPC code. The coded codewords are parsed into different streams using either a bit parser or a group parser. The number of LDPC codewords within one packet is decided by the packet length and the concatenation rules.
Three concatenation rules exist. In the WWiSE approach, described in C. Kose and B. Edwards, “WWiSE Proposal: High throughput extension to the 802.11 Standard,” a contribution to IEEE 802.11, 11-05-0149r2, March 2005 (incorporated herein by reference), only one codeword length and a shortening based scheme is utilized. This approach provides the simplest solution, but the worst performance in terms of extra OFDM padding efficiency. The TGn Sync approach described in S. A. Mujtaba, “TGn Sync Proposal Technical Specification,” a contribution to IEEE 802.11 11-04-889r4, March 2005 (incorporated herein by reference), adopted two code lengths per rate. In order to minimize the extra OFDM symbol padding, shortening and puncturing are used for low data rate transmission. This approach chooses the code length based on the packet length, and always add 1 extra OFDM symbol padding at low rate transmission.